Propylene-based polymers are well known in the art for their usefulness in a variety of applications, including use in packaging, films, and nonwoven fabrics. Often a stabilizer or antioxidant is added to the polymer during the manufacturing or processing process to prevent oxidation of the polymer during storage and use. Nevertheless, plastics derived from propylene-based polymers are known to discolor over time. The discoloration can be readily apparent in end-use applications having large surface area, such as packaging, films, or nonwovens, and may give the end-use product an undesirable appearance.
The discoloration of the polymer can result from the use of antioxidants, such as phenolic antioxidants, that are added to provide processing stability to the polymer. As the phenolic antioxidant functions to capture radicals that are formed on the polymer chain, the phenolic additive degrades and the degradation product is often colored. For example, polymers or articles derived from those polymers where phenolic antioxidants are used as stabilizers are often stored for extended periods of time and are likely exposed to oxides of nitrogen (NOx) gases, such as NOx gases generated from natural gas heaters or gas-powered forklifts. As the phenolic antioxidant captures the radicals that are formed on the polymer as a result of NOx gas exposure, an intense red color arises from the antioxidant that makes the polymer/article appear pink and is referred to as “pinking” of the polymer.
Oxidation, and thus discoloration of the polymer, may also be enhanced by processing conditions such as high heat or excessive melt shearing. Additionally, the presence of other additives such as colorants, UV stabilizers, or slip agents, may also increase the likelihood of discoloration, and may lead to other types of discoloration such as yellowing or browning of the polymer.
U.S. Pat. Nos. 6,881,800 and 7,163,989 and PCT Publications WO 2011/087729; and WO 2011/087730 describe processes and apparatuses for the continuous solution polymerization of olefins including ethylene, propylene, and other olefin commoners. PCT Publication WO 2011/087731 describes the addition of stabilizer to the polymer at a location downstream of the reactor.
U.S. Pat. Nos. 5,844,029; 5,880,191; 5,922,794; and 6,103,798 describe polymer compositions admixed with hydrocarbon amine oxide stabilizer compositions.
U.S. Pat. No. 4,876,300 describes polyolefin compositions stabilized with long chain N,N-dialkylhydroxylamines. U.S. Patent Application Publication No. 2002/0086924 describes stabilized gamma irradiated polyolefins.
There remains a need for propylene-based polymers with improved stability and that have reduced discoloration, for example, reduced pinking, yellowing, and/or browning. There also remains a need for a process to introduce stabilizer into polymer in an effective way that is also compatible with the polymerization and finishing processes for producing the polymer.